Generally, a semiconductor memory device includes a data input unit for inputting data, a core region to process the data transferred from the data input unit, and a data output unit for outputting to an external circuit the processed data from the core region.
Data input pads and data output pads are also included in the semiconductor memory device to support data input and output operations. The data input and output units are connected to data input and output pads, respectively. Recently, a pad through which the data can be inputted and outputted is widely used in order to reduce the number of pads in the semiconductor device.
The data transmission is carried out with a sufficiently large signal, when the data are transferred to the semiconductor memory device; however, when the data processed in the core region are outputted with a weak signal, the data output unit should have a large driving strength in order to pull up and down a load to an external circuit based on the data transferred from the core region. Accordingly, a data driver has to be involved in the data output unit.
Recently, in a case of mobile DRAM, the driving strength of the data driver is controlled by EMRS (Extended Mode Register Set) signal. A first driving strength, a second driving strength, a third driving strength and a fourth driving strength are respectively denoted as “FULL”, “HALF”, QUARTER” and “OCTANT” in the specification. Here, the first driving strength (FULL) means the full driving strength available to drive the data, the second driving strength (HALF) is a half of the full driving strength, the third driving strength (QUARTER) is a quarter dose or one-fourth of the full driving strength, and the fourth driving strength (OCTANT) is an eighth of the full driving strength.
In a case that the driving strength of the data driver is designed to be only the first driving strength (FULL), the second driving strength (HALF), the third driving strength (QUARTER) and the fourth driving strength (OCTANT), the semiconductor memory device does not meet the user's demand precisely. That is, when the user wants to control the data driver with a driving strength which is different from that prescribed in the specification (for example, the user wants to control the data driver with three-fourths of the first driving strength (FULL)), it is not possible to obtain such a driving strength and the variation of the driving strength is limited to a specific option of the prescribed driving strength.